B-cells express surface immunoglobulin (Ig) either with κ (kappa) or λ L chain, a choice which is termed isotype exclusion. The proportion of antibodies containing a κ or λ L chain varies considerably in the different species but in the mouse only a few percent of antibodies express λ. L chain genes are encoded by 2 different loci, the κ or λ L chain loci, and in the mouse there is an extensive number of V(variable)κ genes upstream of 5 J(joining)κ and 1 C(constant region)κ gene. Although the κ locus is over 10-times larger than the λ locus, with more then 100 V genes, this extensive complexity is not regarded as a reason that most mouse antibodies carry a κ L chain. It may be that the mouse κ locus is simply more efficient in DNA rearrangement which is supported by the finding that in the majority of cells with rearranged Vκ the λ locus is still in germline configuration whilst in most cells expressing λ L chain the κ locus is either non-productively rearranged or deleted.
Several mouse strains with silenced κ L chain locus have been described. They were generated by homologous integration of a selectable marker gene in Cκ or targeted removal of Cκ or Jκ (see for example Zou, X. et al., 1995, Eur. J. Immunol 25(8): 2154-2162).
Silencing expression of κ L chain shed light on isotype exclusion and L chain activation and it was concluded that κ and λ expression are separate and independent events. Although homozygous κ−/− mice compensate for the κ deficiency with increased λ production their splenic B-cells and μ+ cells in the bone marrow can be reduced compared to normal mice. This may suggest that λ L chain rearrangement and expression is perhaps a less efficient process. However, despite the lack of κ L chain these mice are healthy and can mount an efficient immune response.
During B-cell development gene segments encoding Ig H chains rearrange first by D to JH recombination at the pro B-cell stage. This is followed by VH to D-JH recombination at the pre B-I stage and if a μ H chain can pair with a surrogate L chain, consisting of VpreB and λ5 protein, this forms a surfaced expressed pre B-cell receptor (pre BCR) at the pre B-II differentiation stage. Cell surface expression of the pre BCR induces proliferation and after several divisions large pre B-II cells differentiate into small resting pre B-II cells. The pre B-II stage with a defined ratio of large and small pre B cells has been identified by surface expression of the IL-2 receptor α chain, CD25. At the transition from pre B-II to immature B cell L chain V-J rearrangement occurs where the surrogate L chain is replaced by κ or λ. At this stage the cells can leave the bone marrow for further differentiation into plasma cells or memory cells in secondary lymphoid organs such as spleen or lymph nodes.